1. Field of the Invention
The present invention relates to an apparatus for detecting and removing additive disturbance on a data transmission channel, and a method therefor, and more particularly, to an apparatus for removing disturbance included in an output signal of a magneto-resistive head sensor, caused by contact with a recording surface of a rotating magnetic disk, and a method therefor.
2. Description of the Related Art
In general, in a data channel where a signal is detected using a magneto-resistive sensor (MR sensor) head which is supported on an air layer formed between a slider assembly and a rotating magnetic disk, heat is generated if the MR sensor ever contacts the recording surface of the rotating magnetic disk. Such heat generated at the contact point raises the temperature of the MR sensor by approximately 1.degree. C. for 50.about.100 nanoseconds. The resistance of the MR sensor is dependent on temperature, and thus increases. The heat generated at the contact point then dissipates into the periphery of the sensor, thereby lowing the resistance back to the original value. Since the MR sensor converts change in magnetic information into a voltage signal, the change in resistance due to the heat acts as an additive disturbance to the output of the MR sensor, resulting in a disturbance mixed with the original data, also known as a thermal asperity transient. The amplitude of such thermal disturbance signal may be four times or more that of the original signal.
Such disturbance to the data signal causes a serious problem for signal detection. In the first stage of the signal detection, it is very difficult to achieve an automatic gain control unit capable of tolerating the disturbance. Even when the automatic gain control unit can perform its own operation during the disturbance, the phase of the signal is disturbed and it is impossible to subsequently fine the exact phase of the signal, in a timing recovery unit. The timing recovery unit does not operate normally over a period of hundreds of or more data symbols, until the amplitude of the disturbance decreases to a negligible level compared to the signal. Also, it takes a long time to find the exact phase. Thus, there are required a method and an apparatus for removing from a playback signal disturbance due to the change in temperature of an MR sensor, caused by the MR sensor contacting the recording surface of the rotating magnetic disk. Here, the disturbance must be eliminated prior to the signal processing through the automatic gain control unit, timing recovery unit and equalization unit, in order to minimize the effect of the disturbance on these circuits.
To solve the problems and meet the above requirement, there has been suggested a method for canceling out the disturbance by adding a corrective feedback signal fed back from the output to the input signal with the disturbance. The corrective feedback signal is an estimation of the disturbance, and is obtained using a non-linear adaptive filter from a differential signal calculated from the envelope of the output signal using an envelope detector and a differentiator installed at the output. However, when estimating the disturbance, only the output signal is used, and the input signal is not monitored at all. As a result, there is a time delay between the start of the disturbance and when the corrective feedback signal begins to compensate for the disturbance, during which the disturbance is not corrected at all.